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Spanning Tree Definition 13  A subset of edges in a graph that:  Span all nodes  Do not create any cycles  This structure is a tree

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802.1 Spanning Tree Approach Elect a bridge to be the root of the tree 2. Every bridge finds shortest path to the root 3. Union of these paths becomes the spanning tree  Bridges exchange Configuration Bridge Protocol Data Units (BPDUs) to build the tree  Used to elect the root bridge  Calculate shortest paths  Locate the next hop closest to the root, and its port  Select ports to be included in the spanning trees

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Definitions 15  Bridge ID (BID) =  Root Bridge: bridge with the lowest BID in the tree  Path Cost: cost (in hops) from a transmitting bridge to the root  Each port on a bridge has a unique Port ID  Root Port: port that forwards to the root on each bridge  Designated Bridge: the bridge on a LAN that provides the minimal cost path to the root  The designated bridge on each LAN is unique

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Determining the Root 16  Initially, all hosts assume they are the root  Bridges broadcast BPDUs:  Based on received BPDUs, each switch chooses:  A new root (smallest known Root ID)  A new root port (what interface goes towards the root)  A new designated bridge (who is the next hop to root) Root IDPath Cost to RootBridge ID

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Bridges vs. Switches 19  Bridges make it possible to increase LAN capacity  Reduces the amount of broadcast packets  No loops  Switch is a special case of a bridge  Each port is connected to a single host Either a client machine Or another switch  Links are full duplex  Simplified hardware: no need for CSMA/CD!  Can have different speeds on each port

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Switching the Internet 20  Capabilities of switches:  Network-wide routing based on MAC addresses  Learn routes to new hosts automatically  Resolve loops  Could the whole Internet be one switching domain? NO